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Date: Tue, 3 Mar 2015 22:15:49 -0800 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Christian Borntraeger <borntraeger@...ibm.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: [PATCH 3.19 171/175] kernel: tighten rules for ACCESS ONCE 3.19-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Borntraeger <borntraeger@...ibm.com> commit 927609d622a3773995f84bc03b4564f873cf0e22 upstream. Now that all non-scalar users of ACCESS_ONCE have been converted to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only work on scalar types. This variant was proposed by Alexei Starovoitov. Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org> --- include/linux/compiler.h | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -447,12 +447,23 @@ static __always_inline void __write_once * to make the compiler aware of ordering is to put the two invocations of * ACCESS_ONCE() in different C statements. * - * This macro does absolutely -nothing- to prevent the CPU from reordering, - * merging, or refetching absolutely anything at any time. Its main intended - * use is to mediate communication between process-level code and irq/NMI - * handlers, all running on the same CPU. + * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE + * on a union member will work as long as the size of the member matches the + * size of the union and the size is smaller than word size. + * + * The major use cases of ACCESS_ONCE used to be (1) Mediating communication + * between process-level code and irq/NMI handlers, all running on the same CPU, + * and (2) Ensuring that the compiler does not fold, spindle, or otherwise + * mutilate accesses that either do not require ordering or that interact + * with an explicit memory barrier or atomic instruction that provides the + * required ordering. + * + * If possible use READ_ONCE/ASSIGN_ONCE instead. */ -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define __ACCESS_ONCE(x) ({ \ + __maybe_unused typeof(x) __var = 0; \ + (volatile typeof(x) *)&(x); }) +#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */ #ifdef CONFIG_KPROBES -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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